A biologist reported the discovery of a gene yesterday that may be a key player in lung cancer, one of the nation's leading killers.

If the discovery is borne out by further studies, it could lead to treatments and diagnostic tests for a disease that is expected to kill 143,000 Americans this year, said Carlo M. Croce, a molecular geneticist at Temple University in Philadelphia and a member of the scientific team that made the discovery.

Mr. Croce is internationally recognized in the rapidly expanding field of molecular genetics, which in the years ahead appears likely to solve a host of medicine's long-standing mysteries.

The lung discovery was one of two major developments announced in the field this week.

In today's New England Journal of Medicine, another team of researchers announced they are zeroing in on a gene that causes some forms of amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, after the famed baseball player killed by ALS. That announcement represents the first significant breakthrough ever in a disease that has defied explanation for more than a century.

Mr. Croce, who made the lung cancer announcement at the opening symposium of the 82nd annual meeting of the American Association for Cancer Research in Houston, said scientists are now "closing in" on the genetic causes of lung cancer and should know "within one or two years" which genes are responsible.

The gene that Mr. Croce's team is focusing on, a gene known as PTP-gamma, is believed to suppress the growth of cancerous tumors in the lung. This gene is missing from the cells of many lung cancer patients, Mr. Croce said, thus allowing cancer to grow unchecked.

Mr. Croce said that several genes are probably involved in the formation of lung cancer but that this one appears to be one of the most important. "If it's not this gene, then it's very close to it," he said.

"Any scientific advance on lung cancer is welcome," said Joann Schellenbach, a spokeswoman for the American Cancer Society. "The survival rates for lung cancer are very grim. Unless this disease is detected very early, there is not much you can do to treat it."

She said that lung cancer is believed to be caused by a combination of genetic flaws and exposure to cancer-causing chemicals found in tobacco, radon and other substances.

In the research on ALS, scientists said they have found the approximate location of a gene responsible for familial, or inherited, ALS on the long arm of chromosome No. 21. This finding is expected to lead to isolating the precise gene and figuring out exactly what it does.

An estimated 20,000 to 30,000 people in the United States have ALS, and 5,000 new cases are diagnosed each year. ALS is caused by the degeneration and death of motor neurons in the brain and spinal cord, which leads to progressive loss of muscle function and eventual paralysis. It kills nearly all of its victims within five years.